1. Field of the Invention
This invention relates in general to signals read from a data storage medium, and more particularly to an apparatus for providing head amplitude characterization using gain loops.
2. Description of Related Art
Recently developed data storage devices, such as magnetic disk drive devices (i.e., hard disk drives), have increased storage capacity and increased data access speed. With these advantages, magnetic disk drive devices have become widely used as auxiliary memory devices for computer systems. More generally, developments in pulse communications related to these improvements in disk drive technology have recently provided increased speed and reliability in a wide range of pulse communications systems.
The primary features of a magnetic disk drive device that affect storage capacity and access speed are the head, the recording medium, the servo mechanism, the signal processing technique used in the read/write channel, and the like. Among these, signal processing techniques utilizing PRML (Partial Response Maximum Likelihood) detection have greatly contributed to the increased storage capacities and high access speeds seen in modern magnetic disk drive devices.
A read channel circuit in a generic read/write channel circuit of a magnetic disk drive device includes components for initial processing of the analog read signal generated by the read/write head of the device. This processing provides automatic gain control (AGC) amplification, filtering, and equalization, as well as analog-to-digital conversion.
Each read/write head generates or senses electromagnetic fields or magnetic encodings on the magnetic disk as areas of magnetic flux. The presence or absence of flux reversals in the electromagnetic fields represents the data stored on the magnetic disk. A flux reversal is a change in the magnetic flux on contiguous areas of the magnetic disk. The presence or absence of magnetic flux reversals correspond to binary 1's and 0's of a diagnostic input signal. To “write” data onto a magnetic disk, electronic components receive data from a host device and translate the data into magnetic encodings. The head transfers the magnetic encodings onto a portion of the magnetic disk. To “read” data from the magnetic disk, the head is positioned adjacent to the portion of the magnetic disk having the desired magnetic encodings. The head senses and transfers the magnetic encodings from the magnetic disk. The electronic components translate the magnetic encodings into the data, which is transferred to the host device. The host device may be a personal computer or other electronic equipment. The electronic components may apply error detection and correction algorithms to ensure accurate storage and retrieval of data from the magnetic disk. To improve data storage densities on disk drives, magneto resistive and inductive read/write heads have been developed with increased sensitivity to sense smaller amplitude magnetic signals and with increased signal discrimination.
Typically, a hard drive reads data by “peak detection”—detecting a voltage peak created when a flux reversal on a magnetic disk passes underneath the read/write head. However, a partial response maximum likelihood (PRML) algorithm has been developed to improve peak detection as densities and rotational speeds increase. PRML is implemented in the disk drive electronics to interpret the magnetic signals sensed by the read/write heads. PRML disk drives read the analog waveforms generated by the magnetic flux reversals stored on the disk. Rather than look for peak values to indicate flux reversals, PRML digitally samples the analog waveform (the “partial response” portion of the algorithm) and applies signal processing methodologies to determine the bit pattern represented by the waveform (the “maximum likelihood” portion of the algorithm). Accordingly, in a PRML data channel, a normalized readback signal amplitude is required for proper data detection. A variable gain amplifier (VGA) typically is used in the analog signal path for scaling of the readback signal. Known PRML channels require an analog envelope detector circuit to sense the amplitude of the incoming readback signal in order to provide gain corrections to the VGA.
Because of material and manufacturing variations, each head will have a different characteristic signal output level. This level must be normalized by adjusting the head amplifier gain so that the amplitude detection circuits will have the same signal margin. In order to properly perform this normalization adjustment, a specially recorded tape, with a precisely known recorded amplitude, must be used.
In some systems, these problems have been solved by providing a gain adjustment on the amplifier, which must be readjusted periodically, and by providing different signal amplitude detection threshold levels for each combination of read/write, speed and density. These threshold levels were fixed, however, and could not change to adjust for media coating type, wear, or signal degradation over time. Also, these fixed threshold values required that the output of each magnetic head be normalized very precisely by adjusting its amplifier gain before amplitude detection can be reliable.
Amplitude sensing is an important part of the read circuitry of tape systems for digital data recording, which record multiple tracks in parallel across the tape. In this type of system, error detection and correction methods are used extensively, such as parity checking across the parallel tracks or parity checking of the data bits in a single track. Loss of amplitude is an important indicator that a track is in error, and the correction methods can then be used to correct the track to avoid having to reposition and read the data again.
PRML electronics are used to calibrate and tune the PRML read/write channel. For example, calibration of the VGA gain is necessary to ensure accurate data detection and to provide insight as to the integrity and operating condition of a read transducer. To calibrate the VGA gain, a readback signal is provided to the VGA in the read channel. A voltage signal associated with a loop gain of the amplification circuitry may then be sensed and compared with a number of control voltage signals that correspond to digital word values. The digital word values are associated with a control voltage signal equal to the sensed voltage signal representing the relative amplitude of the readback signal. Gain characteristics of the VGA may then be determined by applying reference voltage signals associated with corresponding pre-established gain values to the signal input of the gain modifying amplifier. A control voltage signal may be selectively applied to the amplifier for each of the reference voltage signals until the amplifier output voltage signal is substantially equal to a pre-established reference voltage signal.
To simplify calibration procedures, many hard drives include an additional digital to analog converter (DAC) and an additional analog to digital converter (ADC) for diagnostic testing of the read/write channel. These DAC and ADC are in addition to the other digital to analog converters and analog to digital converters used to perform the reading and writing operations in the read/write channel. However, these ADC designs increase the hardware requirements and thus the size and costs of the read/write channel. Further, measuring the amplitude of a head in a storage device, such as a disk drive, is difficult because factors such as temperature change, head wear, etc. adversely affect the accuracy of readback signal amplitude estimates.
A logic controller may be used to control the VGA gain and correlate a known input signal to a selectable ADC output code. However, correlation of the characterization values with the actual VGA gain code from the gain loops is still a problem.
It can be seen that there is a need for an apparatus for providing head amplitude characterization using gain loops.